Autonomous devices communicating using cellular (or other) radio connections are commonly known as the “Internet of Things” (IoT). Typically IoT devices are autonomous and expected to function for extended periods of time without human intervention. It is expected that IoT devices will be used in high numbers, but with each device communicating infrequently and at relatively low data rates. This presents different challenges for system design compared to conventional radio systems having relatively smaller numbers of devices communicating more frequently and at higher data rates.
3GPP are proposing protocols for Narrow Band (NB) operation of IoT devices within the general structure of the cellular standards maintained by 3GPP. In particular, implementations within the LTE standards are being explored which maintain the higher-layers (RLC, MAC, RRC) of the LTE cellular standards, but which utilise different access and control procedures, as well as different physical channels and signals. These protocols are commonly referred to as NB-IoT.
In conventional cellular systems each UE “camps” on a particular carrier, corresponding to its serving cell, and that carrier is utilised for paging and random access requests. To increase capacity additional carriers can be added (as overlapping cells), and the UEs can be distributed across them with different load balancing techniques. While such techniques are also possible in NB-IoT systems, the synchronisation signals (NPSS, NSSS) and broadcast channels (NPBCH, SIBs) occupy approximately 30% of an NB-IoT carrier and duplication of these is wasteful.
A concept of “anchor” and “non-anchor” carriers has been introduced into Release 13 of the NB-IoT standards. An anchor carrier is one which carries all channels (including broadcast channels) as well as synchronisation signals, while a non-anchor carrier does not transmit the common signalling nor the synchronisation signals. A given cell hence has one anchor carrier, and may use one or more non-anchor carriers. A UE can be configured to utilise a non-anchor carrier after RRC Connection Establishment for unicast traffic, in order to relieve the anchor carrier from such traffic. In Release 13, paging and random access procedures are only supported on the anchor carrier.
In order to increase the overall capacity for paging and random access, it was proposed to utilise non-anchor carriers for those procedures, in addition to the anchor carrier. This introduces new challenges regarding load balancing over random access capable carriers and paging capable carriers within the same cell. Indeed, only a fraction of devices in the cell may support random access procedure or paging procedure over non-anchor carriers. Moreover, all carriers may not be equivalent from a performance point of view (e.g., power boost in downlink could be available on only one carrier). A solution is needed to distribute the UEs efficiently both for random access and paging procedures.